It probably has more to do with some hardwired evolutionary mechanism that governs youthful bliss, midlife doldrums and late-life contentment. When plotted over time on a graph, human happiness typically follows a U-shaped curve with a big dip smack-dab in the middle.

The graph for ape happiness, it turns out, curves in a perfect mirror image to the human one. To figure this out, the authors of the recent study compiled data on the moods of more than 500 great apes from zoos and research facilities across North America, Asia and Australia.

“We find it for these creatures that don’t have a mortgage and don’t have to go to work and don’t have marriage and all the other stuff,” Andrew Oswald, an author of the new study, told the Associated Press. “It’s as though the U shape is deep in the biology of humans” rather than a result of our day-to-day experiences.

In most cases, the middle-aged apes studied were the grumpiest, sitting and watching with disdain as the young’uns swung from ropes and the geezers blissfully groomed one another.

The upshot is that it’s perfectly normal — biologically inevitable, even — to experience a dip in happiness during the transition into middle age.

The best strategy for coping: relax and understand it’s perfectly natural. The worst: poo-throwing.